The dynamic behavior of the acidic protons in a hydrated pseudoliquid phase of H3PW12O40. nH(2)O (n < 6) was analyzed by means of P-31 magic-angle spinning (MAS) NMR. With this method the "isolated" acidic protons on heteropolyanions were distinguished from protons in H3O+ and H5O2+. At 173 K, as evidenced by four P-31 MAS NMR peaks, these "isolated" protons were distributed among polyanions so that heteropolyanions directly bonding to 0, 1, 2, and 3 isolated acidic proton(s) coexist. At 298 K, these peaks coalesced, directly demonstrating for the first time the rapid migration of the isolated protons between heteropolyanions.